The invention relates to a digital frequency receiver which in particular allows the identification of voice-frequency signals in telecommunications signalling codes. It can be used in particular in time-division telephone exchanges to receive interchange signals and to receive the signalling transmitted by subscriber keypad sets.
In time-division telephone exchanges, digital receivers are often connected to circuits and lines by the connection network and by a time multiplex link such as a PCM 32-time slot European standard PCM link. The receiver analyses the signals which it receives in these 32 time slots on a time-sharing basis, operating in conjunction with a logic decision circuit.
Many types of known receivers are based on the theory of linear filters defined by finite difference equations. A receiver of this type is described in French Pat. No. 1,603,175 corresponding to U.S. Pat. No. 3,683,162. Such a receiver has the disadvantage of requiring one band-pass filter per frequency to be recognized and it is difficult to adapt it to multiple codes.
Another type of receiver described e.g. in French Pat. No. 2,269,261 consists of a unit which performs frequency translation of the received signal and which detects low-frequency signals.
This is the principle on which the present invention is based. The translation performed is equal to the frequency to be detected. A low-pass filter allows the amplitude of the DC component of the modulated signal to be determined.
For a given received signal X(t), low-pass filtering of the complex signal EQU X'(t)=X(t)e.sup.jwt
is performed to produce a signal whose amplitude is proportional to the energy contained in the spectrum of X(t) in the neighbourhood of the frequency which corresponds to the angular frequency .omega..
In practice, two circuits--a modulation circuit and a filter circuit--are used in parallel:
for multiplying by cos .omega.t and low-pass filtering; and
for multiplying by sin .omega.t and low-pass filtering. The quadratic mean of the two responses is taken.
A primary object of the invention is to produce a compact receiver controlled by a processor and capable of processing a large number of multifrequency codes.